1. Field of the Invention
The present invention is directed to a tone arm control system and more specifically to a tone arm control system utilizing a negative feedback swivel drive.
2. Prior Art
Although many improvements have been made in systems for reproducing sound recorded on disc records, technical deficiencies are still present in some of the systems components so that the average system does not reproduce the recorded information with full fidelity. It has been recognized generally that the weakest link in a sound reproducing system is the tone arm and the cartridge mounted thereon. The major objectives which are desired in tone arms and cartridges are maximum fidelity reproduction of the information recorded on the disc and minimum wear of both the record and the tone arm stylus.
Most record players have tone arms mounted for horizontal swinging movement about a vertical pivot axis located beyond the rim of the turntable. Accordingly, the stylus end of the tone arm described an arc as it tracks the record groove with the result that the tone arm is tangent to the groove for a minimal portion of the tracking. The lack of tangency between the stylus and record groove gives rise to tracking distortions and undesirable stylus to groove side pressures. These conditions are undesirable in playing monaural records but especially undesirable when playing stereo records containing separate audio channels on opposite sides of the record grooves. The reason is that not only are distortions introduced but audio balance is disturbed due to the unbalanced side pressures.
Most record players use a so-called off-set arm having a pick-up cartridge at the end thereof. The pick-up cartridge consists of a cantilevered stylus and a transducer to transform the mechanical deflection of the stylus into an electrical waveform. The pick-up arm is generally pivoted for movement about a vertical axis at one side of the record such that the center line of the pick-up cartridge is at an angle to the line connecting the vertical pivot axis and the tip of the cantilevered stylus. When tracking an unmodulated groove, the cantilevered stylus should maintain a 20.degree. angle within the vertical plane of symmetry of the cartridge so that oscillations due to groove modulations deflect the cantilevered stylus symmetrically with respect to this orientation. By proper proportioning this off-set angle relative to the length of the arm and the point at which the pivot is located, the tracking angle can be kept at a very low value with a reasonably long arm. Under these conditions the stylus and record wear as well as distortions of the signal are minimized.
Stylus deflections which are not a part of the groove modulation can be caused by various effects. Foremost is the skating force which is the lateral component of the frictional drag as exerted by the spinning record on the stylus tip. This force varies with the angle between the tone arm radius and the tangent of the record groove. It depends on the stylus shape and tracking force and, to an extent, on the intensity and spectral distribution of the modulated signal. Another source of horizontal deflection is the friction of the tone arm bearing. In imperfectly balanced tone arms gravity forces can act on the stylus when the record player is not on a level surface. Distortions and wear can be caused by tone arm oscillations at the resonance frequency which is determined by the effective mass of the tone arm and the compliance of the cantilevered stylus and lies usually below 15 Hz. These oscillations are easily excited by external vibrations and shocks.
To minimize or ameliorate these effects, better tone arms are balanced, generate an anti-skating force of the anticipated magnitude, have low-friction bearings and damping means for the tone arm movements. However, even dynamically balanced tone arms, which are insensitive to translational shocks, provide no protection against rotational impulses and may lose track with warped records.
In the past various attempts have been made to eliminate tracking errors by the use of special mechanisms to insure tangency. The best known of these mechanisms involves the use of a straight line track on which the pickup rides. The track is so positioned that the stylus path passes through the center of the record. This eliminates the tracking error but such supports unfortunately have much more friction than the pivoted tone arm thus preventing widespread acceptance of this type of mechanism. Various types of mechanical or electrical servo mechanisms for translating the tone arm have been proposed but the majority of these fail to utilize any feedback control for the servo. The patent to Sinnett et al. (U.S. Pat. No. 2,536,892) discloses broadly the concept of using a servo system to effect accurate tracking of a stylus, the control signal for the system being obtained by means of a capacitive plate arrangement associated with the stylus.
Another type of arrangement for eliminating tracking error is disclosed in applicant's own U.S. Pat. No. 3,005,059 wherein the pick-up cartridge is mounted for rotation on a bobbin or cylindrical disc which is rotated in response to the pivotal movement of the tone arm about the vertical pivot post by means of frictionless rolling connecting means.